Dynatron circuits



Dec. 26, 1933. Q F M 1,941,393

DYNATRON CIRCUITS 7 Filed Sept. 18, 1931 I Fit 12911 506:

' atmim Patented Dec. 26, 1933 w re STATES DYNATRON CIRCUITS Paul 0, Farnham, Boonton, N. J., assignor to Radio FrequencyLaboratories, Incorporated, Boonton, N. J a corporation of New Jersey Application September 18, 1931 Serial No. 563,635

. 11 Qlaims. (01. 179- 171) This invention relates to dynatron circuits and particularly tomethods of and arrangements for introducing a negative capacity into a circuit to neutralize, either entirely or in part, the positive capacity of that circuit.

In the operation of resistance coupled amplifiers, the gain decreases with increasing frequency since the inherent grid-cathode capacity of the amplifier is shunted across the input terminals and. presents a;progressively lower impedance as the-frequency is increased. By introducing a negative capacity intothe tube input circuit to neutralize the positive capacity already present, the gain of the amplifier will be independent of frequency. r 7

An object of the present invention is to provide methods of and circuit arrangements for coun+ teracting or suppressing the capacity of a circuit by the introduction of a negative capacity. Further objects are to provide methods of and circuitarrangements by which av dynatron amplifier will reflect anegative capacity into the input circuit of-the dynatron. Other specificobjects are to provide a resistance coupled amplifier, of oneor more stages, which exhibits some degree of selectivity'in thattheinput capacity of the tube ortubes is neutralized at theirequencyof the desired signal. 7 I These and other objects of the invention will be apparent from the following specification when taken with the accompanying drawing in whichr, w

- V Fig l is a circuit diagram of an amplifier stage which may be operated in accordance with the j invention, and r I c 1 Figs} and 3 are circuit diagrams-oi amplifier circuits embodyingthe.invention. y

The circuit arrangement illustrated. in Fig. '1, differsfrom the conventional tetrode amplifier "having a resistive load R1; in the plate circuit of.

tube in that an external coupling capacity-Cm is provided between the plate and control gridCn. The plate and screen gridGz are energized from appropriate directicurr'ent sources, notshown, and these circuits are by-passed for signal frequencies by condensers C1. I,

As indicated by the legend,'the stage has an effective input capacity C, as viewed looking into theinput terminals of the stage, andthe value c of thiscapacity C'may be determined mathematically. Assuming that, at the frequencyof a sig- 3 nal voltage, the square of the plate load (R23) is negligible in comparison with the square of the reactance of the ooupling capacity an analysis ofthe stage will show that the efiective input capacity is equal to: c.. 1+pA.,

stage functions. as a customary or positive re sistance amplifier, and is equal to: c

- grid, and plate, and Av is the magnitude of the ratio (Without regard to sign) v of plate load resistance RB ,to' the sum of the plate load resistance R and the tube plate .resistance'r The significance of these equations is that,

.due to, the amplifying action of the tube, a ca pacitive reactanceis reflected into the input circuit by the coupling capacity Cm, which reflected capacity always positive when thestage opcrates as a positive resistance amplifier, and which, in the case of the dynatron, will usually be negative since [LAv is normally considerably greater than unity. t

In deriving these equations, the positive capacitivereactance across the input terminals, either the inherent tube capacity alone or that plus added circuit capacity, was ignored as such positive capacity does not enterinto the mathematical analysis for .the value of the effective input capacity due to the coupling capacity Cm. The inherent and any other positive input capacity mustbe added to the reflected input capacity C to obtain the total input capacity of the tube.

Itwill be apparent that, for dynatron opera ticn, the circuit elements may be so chosen that the reflected capacity just neutralizes the positive capacity, thus giving the input impedance 'when'the energizing potentials are such that the 7 negligible compared with The magnitude of the reflected negative capacity is determined by Cm and the plate load RB, since the voltage c in the dynatron plate circuit is dependent upon the plate loa In one satisfactory arrangement for frequencies from 100 to 7000 cycles when using a screen grid tube of the commercial 224 type with 180 volts on the screen and 45 volts on the plate, the coupling capacity Cm was about 50'micromicrofarads, the plate load RB was about 7000 ohms, and the negative capacity reflected into the input circuit was about 150 micromicrofarads.

The negative capacity reflected into the dynatron input circuit may be impressed upon another circuit, as shown in Fig. 2, to neutralize the positive capacity of that circuit, or the dynatron may act itself as the signal amplifier, as shown in Fig. 3.

In Fig, 2, the three tubes 1, 2 and 3 are coupled to the plate resistances 4 and input resistances 5 of adjacent tubes by condensers 6 to constitute a cascaded resistance-coupled amplifier of the usual positive plate resistance type. To neutralize the inherent and positive capacity across the input terminals of tubes 2 and 3, the input circuits of the dynatrons '7 are connected across the grid and cathode terminals of amplifier tubes 2 3, the dynatron stages having resistive plate loads RB and coupling capacities Cm so related as to cancel out all or any desired part of the input capacities of the amplifiers.

While such circuit arrangements are possible when it is desired to employ an amplifier of the usual positive resistance type, a simpler arrangement in which the dynatrons serve also as the amplifiers is usually preferable.

Such an arrangement is illustrated in Fig. 3, in which the tubes 8, 9, 10 have such energizing potentials that the tubes function as dynatron or negative resistance amplifiers. The several tubes have grid resistances 11, and the last tube 10 has a tuned plate circuit comprising an inductance 12 and variable condenser 13, while the preceding stages have plate resistances 14 (corresponding to resistance RB of Fig. 1). The stages are coupled by condensers 15, and in each stage a coupling capacity Cm is provided between the plate and control grid of the dynatron.

The coupling capacity Cm of tube 10 is so chosen that, for the resonant frequency of the tuned circuit 12, 13, i. e., when the plate load impedance is resistive, the inherent input capacity of the final stage is neutralized. At this particular frequency, the plate load of the preceding tube 9 is also a pure resistance, but at other frequencies, the plate load of the, final tube has too low a value of resistive component to neutralize the shunt capacity effects normally present in the input circuit of the final tube. For this reason,

the transmission of frequencies off resonance from the control grid of tube 9 to. the control grid of tube 10 will begreatly decreased, i. e., will be decreased to whatever value it would have with the shunt capacity effects present. This same argument would then apply to the falling off of transmission back through the preceding stages when the impressed frequency differs from the resonant frequency of the final circuit.

Although the plate load of the final tube may have either capacitive or inductive components of reactance for frequencies off resonance and although these reactive components will reflect negative or positive resistance into the input circuit, such effects will be small compared to that of the now unneutralzed shunt capacities in lowering the transmission.

Other applications of the invention will be apparent to those familiar with the design of alternating current networks.

I claim:

1. In the operation of a dynatron stage, the method of introducing a negative capacity into the input circuit of the dynatron, which comprises including a resistive load in the plate circuit, and capacitively coupling the plate and control grid of the dynatron.

2. The method of employing a dynatron stage having input terminals connected to the control grid and cathode thereof to introduce'a negative capacity between two points in a circuit, which comprises connecting across said points the input terminals of the dynatron stage, imparting a resistive plate load' to the dynatron stage, and capacitively coupling the dynatron control grid and plate.

3. Apparatus for producing a negative ca pacity effect, comprising a vacuum tube and meansfor impressing on the elements thereof potentials effective to impart a negative plate resistance to said tube, a resistive load in the anode circuit of said tube, and an external capacity coupling the anode and control grid of said tube.

a. Means for producing a negative capacity effect, comprising a vacuum tube and means for impressing on the elements thereof potentials efifective to impart a negative plate resistance to said tube, a resistive load in the tube plate circuit, and a capacity coupling the plate and. control grid of said tube, said capacity having a reactance substantially greater than the im pedance of said resistive load.

5. The combination with an alternating current circuit, of means for introducing a negative capacity between two points in said circuit; said means comprising a dynatron having the grid and cathode elements thereof connected to the 1 respective points, a resistive load in the plate circuit of said tube, and a capacity between the dynatron plate and grid, said capacity having a reactance substantially in excess of the impedance of said load.

6. The combination with a vacuum tube and associated input and output circuits, of means reducing the capacity effectively across one of the said circuits; said means comprising a dynatron having the input circuit thereof connected across the said circuit of said tube, and means producing a negative capacity across the input circuit of said dynatron.

'7. The combination with a circuit having a capacitive impedance between two points in said circuit, of means for suppressing the effects of said capacitive impedance, said means comprising a dynatron having the grid and cathode terminals thereof connected to the respective points in said circuit, and a resistive load in the plate circuit of said dynatron.

8. In a dynatron circuit, a tube having the elements thereof subjected to energizing potentials effective to impart a negative plate resistance to said tube, and means for establishing a negative capacity between the cathode and a grid element of said tube, said means comprising a resistive plate load and a capacitive coupling between the tube plate and the grid element.

9. The combination with a tube and means energizing the same to function as a dynatron, of a tuned load in the plate circuit of said tube, and a capacitive coupling between the dynatron plate and control grid, said coupling having a reactance of an order substantially greater than the impedance, at resonance, of said tuned load.

10. The combination with a transmission system including a plurality of 'dynatrons cascaded by resistance coupling, of capacity between the plate and'control grid of each dynatron, and a resistive load in the plate circuit of the last dynatron in said system, the square of the resistance of said load being negligible in compari- 'son with the square of the impedance of said 

